Pygmy dinosaur inhabited Bristol's tropical islands

The celebrated Bristol Dinosaur, Thecodontosaurus, has now been shown to live on subtropical islands around Bristol, instead of in a desert on the mainland as previously thought.

This new research could explain the dinosaur's small size (2 m) in relation to its giant (10 m) mainland equivalent, Plateosaurus. Like many species trapped on small islands, such as the 'hobbit' Homo floresiensis of Flores and pygmy elephants on Malta, the Bristol Dinosaur may have been subjected to island dwarfing.

Geological mapping indicates that the islands were quite small in size and, judging by abundant remains of fossil charcoal, were often swept by fires. Thus the pygmy Bristol Dinosaur may have met its death in a wildfire.

Thecodontosaurus is one of the earliest named dinosaurs. Its bones were originally found near what is now Bristol Zoo in 1834 - some time before dinosaurs were recognised as a group. In 1975, the remains of at least 11 other individual dinosaurs were uncovered in a quarry at Tytherington, north of Bristol.

Now, a collaboration between two palaeontologists, Professor John Marshall, a University of Southampton expert on fossil pollen, based at the National Oceanography Centre, Southampton and Bristol University's Dr David Whiteside, an authority on extinct reptiles, has revealed that Thecodontosaurus lived more recently than was previously thought.

Dr Whiteside emphasises that this is 'a unique equal collaboration between a palaeontologist specialising in pollen which are microfossils and a vertebrate palaeontologist working on Triassic reptiles.' He says: 'I can't think of any other scientific paper where the two specialisms were combined to produce a complete paleoenvironmental model which includes the whole community of land animals showing the time and habitat they lived in and how they died.'

Microscopic study of marine algae and fossil pollen shows that, rather than inhabiting the arid uplands of the late Triassic Period, the dinosaurs lived just before the Jurassic Period in a series of lushly vegetated islands around Bristol, the outlines of which can still be seen today in the shape of the land.

Professor John Marshall comments: 'The cave deposits with dinosaurs have been known for over 150 years and are world famous. You would think there would be nothing new to find. But by looking at new deposits with a fresh mind we have been able to radically change the environmental interpretation. The big surprise was discovering that these reptiles did not live on arid uplands but rather on small well-vegetated tropical islands around Bristol around 200 million years ago. It is only the microfossil pollen and algae that can tell us this. The outlines of the islands can still be seen today in the shape of the land.'

Professor Marshall and Dr Whiteside add that the deposits that contain the dinosaurs and other reptiles are very unusual. The bones are found in fossil caves, formed by Triassic rain and seawater dissolving the 350 million-year-old Carboniferous Limestone. The caves then filled with sediments including the dinosaur bones as sea levels rose at the very end of the Triassic Period.

Thecodontosaurus bones have been discovered on both Cromhall Island, north of Bristol and Failand Island, part of which is in the city of Bristol and a short distance inland from the present coast. Geological mapping indicates that the islands the dinosaurs lived on were quite small in size.

The discovery that the Bristol dinosaur lived on very small islands is very important as most researchers have believed that it was a primitive member of the prosauropods, which included some very large animals and existed before the huge sauropods such as Diplodocus of the Jurassic.

'This changes the context which we should view Thecodontosaurus,' says Dr Whiteside. 'It has many similarities to the giant Plateosaurus that lived at the same time and other researchers have not taken into account the rapid changes that take place when large animals are isolated on islands of decreasing size. We believe that the Bristol dinosaur is probably a dwarfed species that derived from the giant Plateosaurus or a very similar animal.'

The study The age, fauna and palaeoenvironment of the Late Triassic fissure deposits of Tytherington, South Gloucestershire, UK is published in Geological Magazine, vol 145, pages 105-147 (2008).